Microporous polyolefin films are widely used as separation membranes for separation or selective permeation of various substances, separator materials, and the like. For example, the microporous polyolefin films are used as a microfiltration membrane, a separator for fuel cells, a separator for capacitors, a base material for a function membrane in which a functional material is filled in the pores to give rise to a new function, and a separator for batteries. Among these applications, the microporous polyolefin films are particularly suitably used as separators for lithium-ion batteries widely employed in notebook-size personal computers, cellular phones, digital cameras, and the like. The reasons include that the microporous polyolefin films are excellent in the mechanical strength and fusing property.
The fusing property refers to a capability securing the battery safety by a separator melting to close its pores to block the battery reaction when the inside of the battery is overheated by an overcharged state or the like. A lower temperature producing the pore closure is considered to have a higher effect on the safety.
The separator is also required to have at least some measure of strength to be able to withstand the tension during the rolling of the battery or due to the rolling of the separator or to prevent short circuiting due to foreign bodies and the like within the battery or film rupture due to impact.
In addition, for recent lithium-ion secondary batteries, the separator is required to have higher porosity for the purpose of the higher power and higher capacity of the batteries.
The separator is also required to have excellent thermal shrinkage characteristics at high temperatures, which include showing excellent results in a high temperature storage test, a high temperature cycle test, an oven test, and the like in the form of a battery. It is generally considered that the separator is preferably low in thermal shrinkage, particularly in thermal shrinkage in the width direction (direction perpendicular to the machine, hereinafter referred to as “TD”), at high temperature. However, the higher strength, fusing property and higher porosity are typically incompatible with the lower TD thermal shrinkage; it has previously been difficult to provide a separator excellent in all of these characteristics.
The separator is also required to be well contained as a roll form thereof into a square-shaped battery container or the like and closely contacted with electrodes.
However, a separator has not previously been present which is excellent in the mountability as a battery roll and thermal shrinkage characteristics at high temperatures as well as having high strength, excellent fusing property and high porosity.
By way of example, patent documents 1 and 2 propose a microporous film in which a low melting point polyethylene such as copolymer polyethylene and low density polyethylene is used to improve the fusing property. However, these methods tend to improve the fusing property, but there is concern that the methods increase the thermal shrinkage, which results in insufficiency in the mountability of the film as a battery roll and performance of the battery in the oven test.
Patent document 3 proposes a microporous film consisting of different laminated films. The method proposed in the document 3 can be said to cause no shrinkage in the TD direction because drawing is not carried out in the TD direction. However, the film provided in the document 3 is a film obtained through a pore forming process by uniaxial stretching only in the length direction (machine direction, hereinafter referred to as “MD”) and represents a microporous film having insufficient TD strength and extreme anisotropy. Thus, there is some apprehension that the film is easily torn in one direction, for example, in a test such as a crush or impact test for the battery.
Patent document 4 proposes a microporous film having high strength and low TD shrinkage, obtained by providing the step of relaxing the shrinkage force in the TD direction. However, it is difficult to sufficiently remove the remaining stress in a slit roll only by the thermal relaxation step. This results in insufficiency in the mountability as a battery roll and performance of the resulting battery in the oven test.
Patent documents 5 to 9 propose obtaining a microporous film having high porosity, high strength and low shrinkage by providing thermal relaxation treatment and heat fixation treatment in a film formation process. However, providing the long-term thermal relaxation/heat fixation treatment step in the film formation process is not only unsubstantial in terms of production but also can be said to result in reduction in the ratio of MD to TD thermal shrinkage and insufficiency in mountability as a battery roll and performance of the battery in the oven test.
As described above, a separator has not previously been present which is excellent in mountability as a battery roll and thermal shrinkage characteristics at high temperatures as well as having high strength, excellent fusing property and high porosity.
Patent Document 1: Japanese Patent No. 3113287
Patent Document 2: Japanese Patent No. 3681720
Patent Document 3: Japanese Patent Laid-Open No. 2005-56851
Patent Document 4: Japanese Patent Laid-Open No. 2001-81221
Patent Document 5: Japanese Patent Laid-Open No. 2003-103624
Patent Document 6: Japanese Patent Laid-Open No. 2003-103625
Patent Document 7: Japanese Patent Laid-Open No. 2003-103626
Patent Document 8: Japanese Patent Laid-Open No. 2003-105121
Patent Document 9: Japanese Patent Laid-Open No. 2003-105122
An object of the present invention is to provide a microporous polyolefin film that can be used as a separator, when set in a battery, providing excellent battery characteristics and a high temperature safety for the battery.